Thursday, 20 March 2014

Shewanella, new pathogenic genus on the marine scene?

The National Oceanic and Atmospheric Administration (NOAA) report the marine environment to be the source of human disease only very rarely. Only a handful of marine taxa are known to cause human disease, and of those the majority are Vibrios. Most sea-borne bacterial infections come from four distinct routes; 1) consumption of seafood, 2) damaged tissue entry, 3) occupational exposure and 4) hobbyist exposure (seashell collectors, aquarium people, etc).
Although unusual circumstances may allow any human pathogen to be linked to ocean, most outbreaks can be classified into 2 groups based on their frequency of association with marine systems. Category 1 includes strains common to the ocean and which almost never cause disease outside it. Category 2 consists of strains only rarely associated with marine exposure, with most of their infections being non-marine.
Category 1 is monopolised by Vibrio, but recent advances in genetic techniques have implied that the genus Shewanella belongs in this group also.
During the mass discovery of novel marine microbe groups (driven by cheapening 16S rRNA gene sequencing) Shewanella expanded from a single species, Shewanella putrefaciens, to more than 60. This genus has been found in seawater, sediments, fish, shellfish and many marine groups; it is also present in terrestrial environments. Many new species were previously classed as members of Pseudomonas.
Since their taxonomic upheaval, Shewanella has been found to be phylogenetically diverse and therefore to have pathogenic members. Shewanella algae (yes, this bacteria called algae) was the first new species firmly shown to infect humans; it opportunistically invaded leg ulcers. Following this study, 90% of strains in human infections were identified fully as being S. algae. This has made it unclear which infections S. putrefaciens is responsible for.
Molecular advances are now suggesting that more and more Shewanella species can be human pathogens. A case of Vibrio vulnificus infection in Japan turned out to be Shewanella haliotis instead; this species was found in a similar case in Thailand. 2011 introduced us to Shewanella xiamenensis as a cause of pancreatic infection and Shewanella oneidensis was found in many clinical assays, but without causal links to any particular infections.
Shewanella is everywhere in the marine biosphere, so there is huge potential for human infection. These bacteria usually enter the body via cuts or contaminated seafood. Data on which marine products can carry Shewanella is poor, but shark, clam and mackerel have all been implicated. This genus can cause blood, liver, skin and ear infections.
The incidences of publications citing Shewanella as a human pathogen are escalating. There is a plethora of weird case of Shewanella including pneumonia from a near-drowning, spleen infection in a diabetic and really nasty gastrointestinal disease. The latter has been reported relatively frequently, with Shewanella being discovered in bloody diarrhoea, faecal swabs and food poisoning outbreaks.
Despite this genus being easily identified due to its unique metabolism, oxidase positivity and hydrogen sulphide producing abilities, most isolates up until now have been lumped together as S. putrefaciens. None of genus has had its antibiotic resistance profiles characterised and the big databases have yet to update their taxonomy according to the recent shakedown of this genus.
I wonder how the virulence and ecology of this genus compares to the infamous Vibrio. Genetics has opened up a whole new, unexplored and highly interesting group to future research. This genus is a member of the normal skin microflora of fish, can cause lesions in trout and is known to spoil caught fish. This genus has been found to have excellent electron-accepting qualities and is thus being considered for applications in bioremediation of polluted environments and electricity generation in microbial fuel cells. However, the pathogenicity of this group must first be considered before it is intentionally grown and introduced into the environment.

One big issue is in the sudden rise of this genus; is this because we have rapidly developed the ability to accurately identify these species, or are anthropogenic factors driving the increased incidence of infections by this group?

Janda, J. M. (2014). Shewanella: a Marine Pathogen as an Emerging Cause of Human Disease. Clinical Microbiology Newsletter36(4), 25-29.


  1. It is interesting how the identification of Shewanella sp. causing human infections has increased to drastically with the development of molecular techniques. Although increasing anthropogenic impacts on the environment (direct or indirect) could be responsible for the increasing occurence of this genus, I think it's rather likely that it might have been there all along and we were not able to identify it until recently; or possibly both explanation are true.

    You mentioned the case in Japan where the infections were thought to have been caused by Vibrio vulnificus, and I wonder if the treatments for these infections were successful when they actually targeted a different bacterial species. The fact that Shewanella haliotis infections were mistaken for V. vulnificus ones show that both species (or perhaps even both genera) show similar pathogenicity to humans, and it can be assumed that they have similar metabolic pathways and susceptibilities to antibiotics. May be V. vulnificus and S. haliotis even interact with each other to infect the host, perhaps one of them weakens the immune response of the host and the other causes the actual infection? It would be also interesting to find out if we falsly blamed the Vibrio sp. for other infections when actually Shewanella sp. were responsible.

    1. Shewanella has probably always been there in the background, until our molecular techniques improved enough to reveal it; measuring future of Shewanella abundance, diversity, etc could give an indication of any anthropogenic influences.
      The Japanese case does not go into much detail and it is only 1 person, as are the other cases mentioned, so these reports cannot really tell us much about Shewanella's metabolism, antibiotic susceptibility, etc. However I do agree with you, Malin, that they do highlight new areas for proper research into Vibrio/Shewanella interactions and physiology.

  2. S. putrefaciens is also capable of producing tetrodotoxin, with the growth and TTX production tightly linked to temperature and salinity. I think it would be a big mistake to intentionally introduce the genus into the environment, particularly considering the mystery shrouding the origins of TTX.

    I wonder whether any changes in temperature or salinity have promoted the growth of other Shewanella species, maybe they were in VBNC states? Although I feel it most likely that it is new technology combined with keen scientific minds to question what they're seeing that has allowed these other species to be identified.

    1. Perhaps the whole mystery around tetrodotoxin stems from us overlooking a previously undetectable microbe group, such as Shewanella. I do not know TTX production is rare or not, but maybe there are members of Shewanella responsible for symbiotic production of TTX.